final Iterator<Entry<R, Map<C, V>>> rowIterator = backingMap.entrySet().iterator();
    @Nullable Entry<R, Map<C, V>> rowEntry;
    Iterator<Entry<C, V>> columnIterator = Iterators.emptyModifiableIterator();

    @Override
    public boolean hasNext() {
      return rowIterator.hasNext() || columnIterator.hasNext();
    }

    @Override
    public Cell<R, C, V> next() {

    Iterator<V> valueIterator;

    Itr() {
      keyIterator = map.entrySet().iterator();
      key = null;
      collection = null;
      valueIterator = Iterators.emptyModifiableIterator();
    }

    abstract T output(@ParametricNullness K key, @ParametricNullness V value);

    @Override
    public boolean hasNext() {

    assertThrows(UnsupportedOperationException.class, () -> iterator.add("a"));
  }

  public void testEmptyModifiableIterator() {
    Iterator<String> iterator = Iterators.emptyModifiableIterator();
    assertFalse(iterator.hasNext());
    assertThrows(NoSuchElementException.class, () -> iterator.next());
    assertThrows(IllegalStateException.class, () -> iterator.remove());
  }

    assertThrows(UnsupportedOperationException.class, () -> iterator.add("a"));
  }

  public void testEmptyModifiableIterator() {
    Iterator<String> iterator = Iterators.emptyModifiableIterator();
    assertFalse(iterator.hasNext());
    assertThrows(NoSuchElementException.class, () -> iterator.next());
    assertThrows(IllegalStateException.class, () -> iterator.remove());
  }